Yes, I tryed that, but when using the external IP a weird error ocurrs.
It's kind of difficult to explain, but the website shows some random content from any of the scripts hosted in that particular virtualhost instead of showing the appropiate info.
Also other websites using different ips get mixed up with the website associated to that ip (EXTERNAL_IP_3)...
For example:
EXTERNAL_IP_1 is associated with www.websiteA.com but, when configuring the upstream with EXTERNAL_IP_3, www.websiteA.com displays the info (the bugged info) of www.example.com (wich is associated to EXTERNAL_IP_3).
Probably that can be solved by using the correct IP for each virtualhost in proxy_pass (instead of http://apache) .. but eitherway, we still have the "bugged content" stuff there.
I KNOW! it's difficult to explain.